Emerging deep ultraviolet light emitting diodes (DUV LEDs) cover the ultraviolet (UV) range down to 210 nanometers (nm), and provide output powers already sufficient for many applications. Additionally, these devices have high modulation frequencies, low noise, flexible form factor and spectral and space power distribution, high internal quantum efficiency, and a potential to achieve high wall plug efficiency. For example, photoluminescence (PL) studies and ray tracing calculations show that the achieved internal quantum efficiency for a 280 nm DUV LED may be quite high, e.g., between fifteen and seventy percent.
However, external quantum efficiency and wall plug efficiency of typical DUV LEDs is below three percent, with the highest efficiencies for 280 nm LEDs and lower efficiencies for LEDs emitting ultraviolet light having shorter wavelengths. Some reasons for the lower external and wall plug efficiencies include very low light extraction efficiency due to internal reflection from the sapphire substrate and sapphire/air interface, and strong absorption in the top low aluminum (Al)-content p-type aluminum gallium nitride (AlGaN) and p-type gallium nitride (GaN) layers. The efficiency of the LEDs is further reduced at higher currents and/or generated powers.
In UV LEDs emitting ultraviolet light having a shorter wavelength, the internal quantum efficiency also drops due to material problems resulting from growth of AlGaN structures with high Al content. Such growth, among other things, is complicated by the low mobility of Al adatoms, which can result in inhomogeneous Al composition and lateral phase separation, as well as high density of threading dislocations and point defects.
One approach to providing a nitride-based light emitting heterostructure that more efficiently generates and extracts light seeks to confine the light generating multiple quantum well structure in an energy “tub.” However, such an approach is currently difficult to implement for short wavelength structures where the aluminum molar fraction is very high.
Controlling doping during the manufacture of many types of devices fabricated with wide band gap semiconductor materials is difficult. In particular, impurity levels for wide band gap semiconductor materials are deep and the activation of the impurities is inefficient, thereby making the doping more difficult to control. For example, FIG. 1 shows an illustrative fraction of activated impurities (Magnesium (Mg)) at 300 Kelvin (K) as a function of the impurity level in Aluminum Gallium Nitride (AlGaN) as shown in the prior art. As illustrated, for a Mg acceptor level in AlGaN of approximately 0.1 electron Volts (eV) above the ceiling of the valence band, only approximately one percent of the impurities are activated and supplying free holes. As a result, the conductivity of p-type AlGaN is severely limited, which is extremely detrimental to the performance of deep ultraviolet light emitting diodes (LEDs).
Polarization doping in GaN-on-AlGaN heterostructures has been shown to lead to the creation of a hole accumulation layer. For example, the polarization charge has been shown to induce a hole sheet density as high as 5×1013 cm−2 at an AlGaN/GaN heterointerface. The transition from a three-dimensional to a two-dimensional hole gas is achieved for hole sheet densities on the order of 1013 cm−2 or higher. At lower hole sheet densities, only a three-dimensional hole accumulation layer may exist. This suggests that a two-dimensional hole gas induced by the polarization charge can be used to reduce the base spreading resistance in AlGaN/GaN-based heterostructure bipolar transistors and/or for p-channel group III nitride-based high electron mobility transistors (HEMTs).
FIG. 2 shows an illustrative band diagram of a metal/AlGaN/GaN heterostructure as shown in the prior art. In this case, the top GaN surface of the heterostructure comprises a nitrogen-terminated surface. In FIG. 2, the calculated two-dimensional charge density distribution includes piezoelectric and spontaneous polarization charges, a metal surface charge, and an accumulation hole charge for the heterostructure. The AlGaN layer comprises an Al molar fraction of approximately 0.25, and does not include donors. The GaN layer comprises an acceptor concentration, Na=1017 cm−3. The horizontal dashed line of FIG. 2 shows the Fermi level, and the holes occupy the energy states above this level. The two-dimensional hole gas provides a large lateral conductivity. However, as illustrated by FIG. 2, the conductance in a direction perpendicular to the two-dimensional hole gas is extremely small. The perpendicular conductance for the heterostructure is limited by the undoped or depleted wide band gap semiconductor layer, e.g., the AlGaN layer.
Carbon has been investigated as an alternative dopant for p-type AlGaN. Ideal delta doping of carbon was demonstrated for gallium arsenide (GaAs). Carbon delta-doped superlattices in GaAs have been successfully grown by chemical beam epitaxy with carbon tetrabromide (CBr4) as the doping source. The carbon in GaAs demonstrated a high electrical activation (3:5×1013 cm−2) and very narrow doping profiles (5° A) due to its high solubility and low diffusivity.
In GaN and AlGaN epilayers, enhancement of the p-type lateral and vertical conductivities has been achieved by employing Mg delta-doping. However, recently, a carbon-doped p-type (0001) plane AlGaN (Al=6% to 50%) with a high hole density has been demonstrated. A stable p-type conduction in the carbon-doped (0001) plane AlGaN was achieved with a large amount of Al (from 1% to 50%), but not in GaN with no Al in the composition. Maximum hole densities for the AlGaN layers with Al compositions of 6%, 10%, 25%, and 50% were approximately (1-3)×1018 cm−3. The “binding energy” of the carbon was approximately 26-30 meV for the carbon-doped p-type AlGaN with 10% of Al. As a result, carbon is a promising acceptor for AlGaN. However, the demonstrated hole densities are still too small for many device applications. Additionally, the expected hole mobilities values are extremely low.